1. Field of the Invention
The present invention relates to a gas sensor element, a method for manufacturing the same, and a gas sensor.
2. Description of the Related Art
A conventional gas sensor is disclosed in Patent Document 1. The gas sensor includes an axially extending gas sensor element for detecting at its front end portion a gas to be measured, a heater for activating the gas sensor element, and a metallic shell for supporting the gas sensor element.
The gas sensor element and the heater have the structure shown in FIG. 11. FIG. 11 is an exploded perspective view of the gas sensor element and the heater. Specifically, the structure is a multilayer structure in which, mainly, a first substrate 101, a heating member 102, a second substrate 103, a first electrode 104, a first solid electrolyte member 105, a second electrode 106, an insulation layer 107, a third electrode 108, a second solid electrolyte member 109, a fourth electrode 110, and a protection layer 111 are sequentially arranged in layers. The protection layer 111 includes a reinforcement member 112 having an insertion hole 112a which extends therethrough in the laminating direction, and an electrode protection member 113a inserted into the insertion hole 112a. The reinforcement member 112 prevents warpage and enhances the strength of a gas sensor element 300 and a heater 200. The electrode protection member 113a is a porous member which protects the fourth electrode 110 (specifically, a fourth electrode portion 110a) from becoming poisoned while maintaining gas communication between the fourth electrode 110 and the atmosphere.
The gas sensor is manufactured as follows. First, a green electrode protection member 113a, which is the electrode protection member 113a before firing, is inserted into the insertion hole 112a of a green reinforcement member 112, which is the reinforcement member 112 before firing. A green protection layer 111 is thus formed, which is the protection layer 111 before firing. Subsequently, a first green substrate 101, which is the first substrate 101 before firing, a first green solid electrolyte member 105, which is the first solid electrolyte member 105 before firing, a second green solid electrolyte member 109, which is the second solid electrolyte member 109 before firing, and the like, together with the green protection layer 111, are arranged in layers, thereby yielding a laminate. Next, the laminate is subjected to resin removal firing, and then main firing, whereby the gas sensor element 300 and the heater 200 are obtained in an integrated fashion. Then, the gas sensor element 300 and the heater 200 are attached to a metallic shell and the like, thereby yielding a gas sensor.
The thus-obtained gas sensor is attached to, for example, an exhaust system such as an exhaust pipe of an engine and is used to detect a gas to be measured which is contained in exhaust gas.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2003-294687
3. Problems to be Solved by the Invention
The above-mentioned conventional gas sensors are at risk of increased variation in gas detection performance among respective sensors. Consequently, in some cases, yield has been insufficient.
Specifically, as shown in FIG. 13, the present inventors have found that, in the gas sensor element, a crack CR tends to occur in the second solid electrolyte member 109, which is formed from the second green solid electrolyte member 109 by firing. Gas sensors having the crack CR are at risk of increased variation in gas detection performance among respective sensors. Thus, in some cases, production yield has been insufficient. This is because even when other requirements are satisfied, occurrence of the crack CR causes a reduction in electric potential between the electrodes developed by a gas to be measured.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to reduce variation in gas detection performance among gas sensors and to enable manufacture of gas sensors at high yield.